Global warming, the
phenomenon of increasing average air temperatures near the surface of Earth
over the past one to two centuries. Climate scientists have since the mid-20th
century gathered detailed observations of various weather phenomena (such as temperatures,
precipitation, and storms) and of related influences on climate (such as ocean
currents and the atmosphere’s chemical composition). These data indicate that
Earth’s climate has changed over almost every conceivable timescale since the
beginning of geologic time and that the influence of human activities since at
least the beginning of the Industrial Revolution has been deeply woven into the
very fabric of climate change.
Giving voice to a
growing conviction of most of the scientific community, the Intergovernmental
Panel on Climate Change (IPCC) was formed in 1988 by the World Meteorological
Organization (WMO) and the United Nations Environment Program (UNEP). In 2013
the IPCC reported that the interval between 1880 and 2012 saw an increase in
global average surface temperature of approximately 0.9 °C (1.5 °F). The
increase is closer to 1.1 °C (2.0 °F) when measured relative to the
preindustrial (i.e., 1750–1800) mean temperature.
The IPCC stated that
most of the warming observed over the second half of the 20th century could be
attributed to human activities. It predicted that by the end of the 21st
century the global mean surface temperature would increase by 0.3 to 4.8 °C
(0.5 to 8.6 °F) relative to the 1986–2005 average. The predicted rise in
temperature was based on a range of possible scenarios that accounted for
future greenhouse gas emissions and mitigation (severity reduction) measures
and on uncertainties in the model projections. Some of the main uncertainties
include the precise role of feedback processes and the impacts of industrial
pollutants known as aerosols which may offset some warming.
Many climate scientists
agree that significant societal, economic, and ecological damage would result
if global average temperatures rose by more than 2 °C (3.6 °F) in such a short
time. Such damage would include increased extinction of many plant and animal
species, shifts in patterns of agriculture, and rising sea levels. The IPCC
reported that the global average sea level rose by some 19–21 cm (7.5–8.3
inches) between 1901 and 2010 and that sea levels rose faster in the second
half of the 20th century than in the first half. It also predicted, again
depending on a wide range of scenarios, that by the end of the 21st century the
global average sea level could rise by another 26–82 cm (10.2–32.3 inches)
relative to the 1986–2005 average and that a rise of well over 1 metre (3 feet)
could not be ruled out.
The scenarios referred
to above depend mainly on future concentrations of certain trace gases, called
greenhouse gases, that have been injected into the lower atmosphere in
increasing amounts through the burning of fossil fuels for industry,
transportation, and residential uses. Modern global warming is the result of an
increase in magnitude of the so-called greenhouse effect, a warming of Earth’s
surface and lower atmosphere caused by the presence of water vapour, carbon
dioxide, methane, nitrous oxides, and other greenhouse gases. In 2014 the IPCC
reported that concentrations of carbon dioxide, methane, and nitrous oxides in
the atmosphere surpassed those found in ice cores dating back 800,000 years. Of
all these gases, carbon dioxide is the most important, both for its role in the
greenhouse effect and for its role in the human economy. It has been estimated
that, at the beginning of the industrial age in the mid-18th century, carbon
dioxide concentrations in the atmosphere were roughly 280 parts per million
(ppm). By the middle of 2014, carbon dioxide concentrations had briefly reached
400 ppm, and, if fossil fuels continue to be burned at current rates, they are
projected to reach 560 ppm by the mid-21st century—essentially, a doubling of
carbon dioxide concentrations in 300 years.
A vigorous debate is in
progress over the extent and seriousness of rising surface temperatures, the
effects of past and future warming on human life, and the need for action to
reduce future warming and deal with its consequences. This article provides an
overview of the scientific background and public policy debate related to the
subject of global warming. It considers the causes of rising near-surface air
temperatures, the influencing factors, the process of climate research and
forecasting, the possible ecological and social impacts of rising temperatures,
and the public policy developments since the mid-20th century. For a detailed
description of Earth’s climate, its processes, and the responses of living
things to its changing nature, see climate. For additional background on how
Earth’s climate has changed throughout geologic time, see climatic variation
and change. For a full description of Earth’s gaseous envelope, within which
climate change and global warming occur, see atmosphere.
Climatic variation
since the last glaciation
Global warming is
related to the more general phenomenon of climate change, which refers to
changes in the totality of attributes that define climate. In addition to
changes in air temperature, climate change involves changes to precipitation
patterns, winds, ocean currents, and other measures of Earth’s climate.
Normally, climate change can be viewed as the combination of various natural
forces occurring over diverse timescales. Since the advent of human civilization,
climate change has involved an “anthropogenic,” or exclusively human-caused,
element, and this anthropogenic element has become more important in the
industrial period of the past two centuries. The term global warming is used
specifically to refer to any warming of near-surface air during the past two
centuries that can be traced to anthropogenic causes.
To define the concepts
of global warming and climate change properly, it is first necessary to
recognize that the climate of Earth has varied across many timescales, ranging
from an individual human life span to billions of years. This variable climate
history is typically classified in terms of “regimes” or “epochs.” For
instance, the Pleistocene glacial epoch (about 2,600,000 to 11,700 years ago) was
marked by substantial variations in the global extent of glaciers and ice
sheets. These variations took place on timescales of tens to hundreds of
millennia and were driven by changes in the distribution of solar radiation
across Earth’s surface. The distribution of solar radiation is known as the
insolation pattern, and it is strongly affected by the geometry of Earth’s
orbit around the Sun and by the orientation, or tilt, of Earth’s axis relative
to the direct rays of the Sun.
Worldwide, the most
recent glacial period, or ice age, culminated about 21,000 years ago in what is
often called the Last Glacial Maximum. During this time, continental ice sheets
extended well into the middle latitude regions of Europe and North America,
reaching as far south as present-day London and New York City. Global annual
mean temperature appears to have been about 4–5 °C (7–9 °F) colder than in the
mid-20th century. It is important to remember that these figures are a global
average. In fact, during the height of this last ice age, Earth’s climate was
characterized by greater cooling at higher latitudes (that is, toward the
poles) and relatively little cooling over large parts of the tropical oceans
(near the Equator). This glacial interval terminated abruptly about 11,700 years
ago and was followed by the subsequent relatively ice-free period known as the
Holocene Epoch. The modern period of Earth’s history is conventionally defined
as residing within the Holocene. However, some scientists have argued that the
Holocene Epoch terminated in the relatively recent past and that Earth
currently resides in a climatic interval that could justly be called the
Anthropocene Epoch—that is, a period during which humans have exerted a
dominant influence over climate.
Though less dramatic
than the climate changes that occurred during the Pleistocene Epoch,
significant variations in global climate have nonetheless taken place over the
course of the Holocene. During the early Holocene, roughly 9,000 years ago,
atmospheric circulation and precipitation patterns appear to have been
substantially different from those of today. For example, there is evidence for
relatively wet conditions in what is now the Sahara Desert. The change from one
climatic regime to another was caused by only modest changes in the pattern of
insolation within the Holocene interval as well as the interaction of these
patterns with large-scale climate phenomena such as monsoons and El
Niño/Southern Oscillation (ENSO).
During the middle
Holocene, some 5,000–7,000 years ago, conditions appear to have been relatively
warm—indeed, perhaps warmer than today in some parts of the world and during
certain seasons. For this reason, this interval is sometimes referred to as the
Mid-Holocene Climatic Optimum. The relative warmth of average near-surface air
temperatures at this time, however, is somewhat unclear. Changes in the pattern
of insolation favoured warmer summers at higher latitudes in the Northern
Hemisphere, but these changes also produced cooler winters in the Northern Hemisphere
and relatively cool conditions year-round in the tropics. Any overall
hemispheric or global mean temperature changes thus reflected a balance between
competing seasonal and regional changes. In fact, recent theoretical climate
model studies suggest that global mean temperatures during the middle Holocene
were probably 0.2–0.3 °C (0.4–0.5 °F) colder than average late 20th-century
conditions.
Over subsequent
millennia, conditions appear to have cooled relative to middle Holocene levels.
This period has sometimes been referred to as the “Neoglacial.” In the middle
latitudes this cooling trend was associated with intermittent periods of
advancing and retreating mountain glaciers reminiscent of (though far more
modest than) the more substantial advance and retreat of the major continental
ice sheets of the Pleistocene climate epoch.
Causes of global
warming
The greenhouse effect
The average surface
temperature of Earth is maintained by a balance of various forms of solar and
terrestrial radiation. Solar radiation is often called “shortwave” radiation
because the frequencies of the radiation are relatively high and the
wavelengths relatively short—close to the visible portion of the
electromagnetic spectrum. Terrestrial radiation, on the other hand, is often
called “longwave” radiation because the frequencies are relatively low and the
wavelengths relatively long—somewhere in the infrared part of the spectrum.
Downward-moving solar energy is typically measured in watts per square metre.
The energy of the total incoming solar radiation at the top of Earth’s
atmosphere (the so-called “solar constant”) amounts roughly to 1,366 watts per
square metre annually. Adjusting for the fact that only one-half of the
planet’s surface receives solar radiation at any given time, the average
surface insolation is 342 watts per square metre annually.
The amount of solar
radiation absorbed by Earth’s surface is only a small fraction of the total
solar radiation entering the atmosphere. For every 100 units of incoming solar radiation,
roughly 30 units are reflected back to space by either clouds, the atmosphere,
or reflective regions of Earth’s surface. This reflective capacity is referred
to as Earth’s planetary albedo, and it need not remain fixed over time, since
the spatial extent and distribution of reflective formations, such as clouds
and ice cover, can change. The 70 units of solar radiation that are not
reflected may be absorbed by the atmosphere, clouds, or the surface. In the
absence of further complications, in order to maintain thermodynamic
equilibrium, Earth’s surface and atmosphere must radiate these same 70 units
back to space. Earth’s surface temperature (and that of the lower layer of the
atmosphere essentially in contact with the surface) is tied to the magnitude of
this emission of outgoing radiation according to the Stefan-Boltzmann law.
Earth’s energy budget
is further complicated by the greenhouse effect. Trace gases with certain
chemical properties—the so-called greenhouse gases, mainly carbon dioxide (CO2),
methane (CH4), and nitrous oxide (N2O)—absorb some of the infrared radiation
produced by Earth’s surface. Because of this absorption, some fraction of the
original 70 units does not directly escape to space. Because greenhouse gases
emit the same amount of radiation they absorb and because this radiation is
emitted equally in all directions (that is, as much downward as upward), the
net effect of absorption by greenhouse gases is to increase the total amount of
radiation emitted downward toward Earth’s surface and lower atmosphere. To
maintain equilibrium, Earth’s surface and lower atmosphere must emit more
radiation than the original 70 units. Consequently, the surface temperature
must be higher. This process is not quite the same as that which governs a true
greenhouse, but the end effect is similar. The presence of greenhouse gases in
the atmosphere leads to a warming of the surface and lower part of the
atmosphere (and a cooling higher up in the atmosphere) relative to what would
be expected in the absence of greenhouse gases.
It is essential to
distinguish the “natural,” or background, greenhouse effect from the “enhanced”
greenhouse effect associated with human activity. The natural greenhouse effect
is associated with surface warming properties of natural constituents of
Earth’s atmosphere, especially water vapour, carbon dioxide, and methane. The
existence of this effect is accepted by all scientists. Indeed, in its absence,
Earth’s average temperature would be approximately 33 °C (59 °F) colder than
today, and Earth would be a frozen and likely uninhabitable planet. What has
been subject to controversy is the so-called enhanced greenhouse effect, which
is associated with increased concentrations of greenhouse gases caused by human
activity. In particular, the burning of fossil fuels raises the concentrations
of the major greenhouse gases in the atmosphere, and these higher
concentrations have the potential to warm the atmosphere by several degrees
Radiative forcing
In light of the
discussion above of the greenhouse effect, it is apparent that the temperature
of Earth’s surface and lower atmosphere may be modified in three ways: (1)
through a net increase in the solar radiation entering at the top of Earth’s
atmosphere, (2) through a change in the fraction of the radiation reaching the
surface, and (3) through a change in the concentration of greenhouse gases in
the atmosphere. In each case the changes can be thought of in terms of
“radiative forcing.” As defined by the IPCC, radiative forcing is a measure of
the influence a given climatic factor has on the amount of downward-directed
radiant energy impinging upon Earth’s surface. Climatic factors are divided
between those caused primarily by human activity (such as greenhouse gas
emissions and aerosol emissions) and those caused by natural forces (such as
solar irradiance); then, for each factor, so-called forcing values are
calculated for the time period between 1750 and the present day. “Positive
forcing” is exerted by climatic factors that contribute to the warming of
Earth’s surface, whereas “negative forcing” is exerted by factors that cool
Earth’s surface.
On average, about 342
watts of solar radiation strike each square metre of Earth’s surface per year,
and this quantity can in turn be related to a rise or fall in Earth’s surface
temperature. Temperatures at the surface may also rise or fall through a change
in the distribution of terrestrial radiation (that is, radiation emitted by
Earth) within the atmosphere. In some cases, radiative forcing has a natural
origin, such as during explosive eruptions from volcanoes where vented gases
and ash block some portion of solar radiation from the surface. In other cases,
radiative forcing has an anthropogenic, or exclusively human, origin. For
example, anthropogenic increases in carbon dioxide, methane, and nitrous oxide
are estimated to account for 2.3 watts per square metre of positive radiative
forcing. When all values of positive and negative radiative forcing are taken
together and all interactions between climatic factors are accounted for, the
total net increase in surface radiation due to human activities since the
beginning of the Industrial Revolution is 1.6 watts per square metre.
The influences of human
activity on climate
Human activity has
influenced global surface temperatures by changing the radiative balance
governing the Earth on various timescales and at varying spatial scales. The
most profound and well-known anthropogenic influence is the elevation of
concentrations of greenhouse gases in the atmosphere. Humans also influence
climate by changing the concentrations of aerosols and ozone and by modifying
the land cover of Earth’s surface.
Greenhouse gases
As discussed above,
greenhouse gases warm Earth’s surface by increasing the net downward longwave
radiation reaching the surface. The relationship between atmospheric
concentration of greenhouse gases and the associated positive radiative forcing
of the surface is different for each gas. A complicated relationship exists
between the chemical properties of each greenhouse gas and the relative amount
of longwave radiation that each can absorb. What follows is a discussion of the
radiative behaviour of each major greenhouse gas
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